1. Field of the Invention
The present invention relates to a photoelectric smoke sensor for outputting smoke-density data (which is an analog value corresponding to a smoke density), in particular, a photoelectric smoke sensor having a function of correcting a detection value which changes over time due to contamination of detection means.
2. Description of the Related Art
The following photoelectric smoke sensor is conventionally known. The photoelectric smoke sensor includes a light-emitting element and a light-receiving element. Scattered light of light emitted from the light-emitting element is detected by the light-receiving element provided in a labyrinth. In this manner, the photoelectric smoke sensor detects smoke.
In the photoelectric smoke sensor as described above, a value detected by the light-receiving element corresponding to detection means changes over time due to contamination occurring in the labyrinth. A technology for correcting a sensitivity has been proposed so as to more precisely detect a smoke density even when the aforementioned change over time occurs (for example, see Japanese Patent Application Laid-open No. 8-255291 (pages 2 and 3 and FIGS. 5 and 6)).
A correction method for a smoke sensor, which is described in Japanese Patent Application Laid-open No. 8-255291 cited above, includes a first step of obtaining a difference between a previous zero-point value of the smoke sensor and a newly measured zero-point value, a second step of correcting the zero-point value to the newly obtained value when the difference is within a correction limit width, a third step of setting a test warning point value to a value corrected by the difference, and a fourth step of correcting a conversion characteristic between a light-receiving amount and the smoke density to a conversion characteristic obtained by connecting the corrected zero-point value and the corrected test warning point value.
According to the method of correcting a sensitivity of the smoke sensor, the conversion characteristic (conversion formula) between the light-receiving amount of the smoke sensor and the smoke density is corrected to a conversion formula obtained by translating the conversion formula in an initial state. Then, according to the corrected conversion formula, the light-receiving amount received by the light-receiving element is converted into an analog value corresponding to the smoke density.
Factors of the change generated in the detection value of the light-receiving element over time include the contamination of an inner wall of the labyrinth in which the light-receiving element is provided and the contamination of the light-emitting element or the light-receiving element.
When the contamination occurs in the labyrinth, the amount of reflection (noise level) of the light emitted from the light-emitting element is increased by a predetermined amount. Specifically, in the environment with the same smoke density, the amount of light received by the light-receiving element is increased by a predetermined amount after the contamination occurs in the labyrinth as compared with that before the contamination occurs. Therefore, for a characteristic function of the light-receiving amount corresponding to smoke-density data, a detection level for the light-receiving amount is shifted upward after the contamination occurs as compared with that before the occurrence of the contamination.
Therefore, after the contamination occurs, the conversion formula is corrected to be translated so that the detection level for the light-emitting amount becomes higher. In this manner, the conversion formula suitable for a state of the contamination can be obtained.
On the other hand, when the contamination of the light-emitting element or the light-receiving element occurs, the detection value of the light-receiving element is reduced at a predetermined rate. Therefore, a slope of a straight line of the characteristic function of the light-receiving amount corresponding to the smoke-density data becomes lower as compared with that before the contamination occurs.
Specifically, as in the related art, with the conversion formula obtained by translating the conversion formula obtained before the occurrence of the contamination, a correction suitable for the state of the contamination of the light-emitting element or the light-receiving element cannot be performed.